Gas and ash emissions associated with the 2010–present activity of Sinabung Volcano, Indonesia

Abstract

Sinabung Volcano (Sumatra, Indonesia) awoke from over 1200years of dormancy with multiple phreatic explosions in 2010. After a period of quiescence, Sinabung activity resumed in 2013, producing frequent explosions, lava dome extrusion, and pyroclastic flows from dome and lava flow collapses, becoming one of the world's most active volcanoes and displacing over 20,000 citizens. This study presents a compilation of the geochemical datasets collected by the Indonesian Center for Volcanology and Geological Hazard Mitigation (CVGHM) from 2010–current (2016), which provides insights into the evolution of the eruption. Based on observations of SO2 emissions, ash componentry, ash leachate chemistry, and bulk ash geochemistry, the eruption can be split into six distinct geochemical phases. The initial stage of phreatic summit explosions (phase A) occurred from August–September 2010, during which background SO2 emissions averaged ~550±180t/d (1s.d.). An eruptive pause (phase B) starting in October 2010 abruptly ended in September 2013 with a resumption of conduit-clearing eruptions (phase C). This third phase had a relatively modest background SO2 emission rate (avg. ~430±310t/d) and produced ash consisting of accidental ejecta with high S/Cl leachate molar ratios (12.0±8.2, max 34), suggestive of deep-sourced magma and/or the incorporation of hydrothermal sulfur-bearing phases. Lava extrusion at the summit (phase D) began in mid-December 2013, and was accompanied by relatively low SO2 emission rates (360±200t/d) and lower, but variable, S/Cl leachate ratios (6.3±8.5). The most intense phase of the eruption (phase E) occurred from mid-January to late February 2014 following a major lava dome collapse. This period included increased lava extrusion rates, dozens of large eruptions per day, high SO2 emission rates (average: 1680±1070t/d, peak: ~3800t/d), and a dramatic drop in S/Cl ash leachates to ratios (average 1.4±0.5), consistent with increased degassing from shallow magma and the clearing out of sulfurous phases from the old hydrothermal system. From March 2014 through the time of writing (September 2016), Sinabung settled into a relatively steady state of lower activity (phase F). Ash emissions now consist of dominantly juvenile andesitic-dacitic material with low S/Cl leachate ratios (average 1.1±0.6). In August 2016, SO2 emissions started being measured in a continuous manner using a network of permanent scanning DOAS instruments. Background SO2 emission rates average 450±290t/d for the entirety of phase F, but have been progressively decreasing to an average of ~250–300t/d since June 2016. This long-term gradual decline in SO2 emission rates at Sinabung since early 2014 is consistent with an apparent decrease in magma supply and lava effusion rates. Our conceptual degassing model suggests that large explosions and pyroclastic flows could continue in the near-term owing to conduit plugging and dome collapses, remaining a major threat until the magma supply rate decreases further and the eruption ends.